The present invention relates generally to a system and method for controlling the operation of an electric servo in a cruise control system. More particularly, in a cruise control system that incorporates an electric servo motor for moving an element to modify the speed of the vehicle's engine, apparatus and a method is included for detecting when the servo motor is running in a stall position so the motor's operation can be modified to prevent damage to the motor and gear reduction system.
Prior cruise control systems have either used, for the most part, pneumatic (vacuum) actuated elements or electric motors as the "servo" mechanism. The servo connects to and effects movement of an engine element for varying the speed of the engine, and thereby controls vehicle traveling speed. Typically, the engine element controlled by the servo is the engine's throttle valve or linkage connected to the throttling valve, although it may also be used on an injected system. Examples of prior cruise control systems employing vacuum servo actuation of the throttle valve are disclosed in U.S. Pat. Nos. 3,455,411, 3,575,256, 3,946,707 and 4,352,403. Although effective to control the speed of many motor vehicles, these vacuum servo actuated cruise control systems suffer from a lack of adequate response to some given motor vehicle speed conditions, such as found in highly uneven terrain. This problem is exacerbated as internal combustion engines become smaller, because the available vacuum becomes more limited. Since these smaller engines typically operate at higher RPM and, therefore, larger throttle openings, the negative manifold pressure available to operate the vacuum servo will be less, which necessitate the use of an even larger vacuum servo on a smaller engine. Further, a vacuum servo actuator will be demanding vacuum at a time when the engine's negative manifold pressure is decreasing. This problem can be overcome through employment of an auxiliary vacuum pump, but the solution increases cost, both in terms of the added parts and in terms of the labor required for the installation time.
Motor vehicles can have many locations to which the vacuum source for the vacuum servo motor can be attached. Unfortunately, the most convenient location (and the one typically used to save time) is near the brake system, because the vacuum source is generally the most reliable, and because the manifold is provided with a convenient opening for a vacuum line at this location. However, this location does not provide a hook-up conducive to proper and efficient operation of the system. A hook-up which uses a vacuum-operated brake system to power the cruise control system causes a deterioration in both brake operation and cruise control operation.
Pressure servo actuators are another type of servo which has been contemplated, but these have found very rare application in conventional motor vehicles due to the lack of an available positive pressure source in today's automobiles. This can be supplied by a separate pump, but once again cost and installation time are increased.
Many of the problems identified above can be solved or at least mitigated by employing an electric servo motor in the cruise control system for moving the throttle valve (or other element). Not only is the size of electric servo motors much smaller, they can respond more quickly and reliably in uneven terrain. Further, the electric current supply can be made relatively independent of the operating limitations of the engine (versus dependence on the engine speed or brake vacuum pressure), the electric motors are inexpensive, and location of the motor is more flexible because the motor only need be connected to the power source by a wire, versus a vacuum hose.
However, electric motor servo cruise control systems are not without their own problems. One such problem concerns protection of the DC motor when the movable element (i.e., engine throttle valve) is moved to one of its extreme positions. If the electronics controlling the servo motor direct the servo to move the throttle valve toward an increase in speed, the motor and connecting gears on the servo are engaged in the direction which will move the throttle valve toward a more open position. The movement will continue until the desired speed is reached and the electronics direct the servo to stop or to reverse. If the throttle valve is moved to its full throttle position, but the electronics direct the servo to keep increasing (i.e. when on a very steep incline), the DC motor will attempt to continue movement as long as a drive current is supplied thereto. This can result in damage to the gearing or linkage that connects the servo to the throttle valve, or damage to the motor itself. Present techniques employ an electromechanical limit switch that is tripped to cut off the servo motor when the throttle valve reaches its full throttle position or idle position.
However, the designs of throttles in today's motor vehicles differ widely in the distances of throttle valve travel from idle to full throttle position. Some have a linear movement of approximately one inch, while others may be 1.75 inches and beyond. This wide range of throttle movement requires the position of each electromechanical limit switch to be set and adjusted at the time each cruise control system is installed. This can significantly increase the cost of the cruise control system, as well as affect its efficiency and operation. Cost is affected in terms of increased parts and the extra labor required for installation, because the limit switch must be adjusted for every model type engine. Even in a factory installation the wide varieties of engine models encountered during the assembly process requires a manufacturer to keep on hand large stocks of spare parts in a wide variety of servos set to different limits, it if was attempted to use non-adjustable limit switches. Efficiency can be affected when the limit switch is installed incorrectly, which may unnecessarily limit full throttle travel or allow excessive travel, which will damage linkage.
Similarly, the idle position must also be detected by the DC servo motor so the motor will not attempt to drive beyond the position corresponding to throttle idle, again possibly damaging the linkage, the motor, or both. Electromechanical limit switches are again used today to detect this limit, with the same problems described above.
Accordingly, it can be seen that there is a need for apparatus capable of detecting the limits of operational travel of an electric motor actuator when used in cruise control systems, but one that is not encumbered by the necessity of having to adjust the limit means individually for each motor vehicle installation.